1. Field of the Invention
The present invention relates to a touch switch including an electric capacitance detection circuit capable of detecting an electric capacitance change caused when a human body approaches or contacts a conductive layer.
2. Description of the Related Art
A conventional touch switch can detect an electric capacitance change caused in the vicinity of a panel surface when a finger comes near to or contacts the panel surface.
FIG. 1 illustrates a conventional touch switch 100 including an insulating base resin layer 10, a metallic layer 12 formed on a reverse surface of the insulating base resin layer 10, and an electric capacitance detection circuit 14 electrically connected to the metallic layer 12 (refer to Japanese Patent Application Laid-open Publication No. 2005-228563 or Japanese Patent Application Laid-open Publication No. Hei 11-136116). FIG. 2 illustrates a conventional touch switch 200 including a metallic layer 16 and an electric capacitance detection circuit 18 electrically connected to the metallic layer 16 (refer to Japanese Patent Application Laid-open Publication No. 2004-103285).
According to the touch switch 100 shown in FIG. 1, when a person places their finger near or on the base resin layer 10, an electric capacitance change occurs between the finger and the metallic layer 12 formed on the reverse surface of the base resin layer 10. The electric capacitance detection circuit 14 connected to the metallic layer 12 can detect the electric capacitance change.
According to the touch switch 200 shown in FIG. 2, when a user's finger directly contacts the metallic layer 16, an electric capacitance change occurs and the electric capacitance detection circuit 18 connected to the metallic layer 16 can detect the electric capacitance change.
In other words, the touch switch 100 shown in FIG. 1 indirectly detects an electric capacitance change via the base resin layer 10. When the distance between a finger and the metallic layer 12 is large, the change in the electric capacitance that must be detected by the touch switch 100 is small.
The change in the electric capacitance when a finger contacts the base resin layer 10 is also small when the base resin layer 10 is relatively thick, i.e., when the distance between a finger and the metallic layer 12 is large. When the electric capacitance change is small, it is difficult to determine whether a user's finger has approached or contacted the base resin layer 10. In other words, the switch sensitivity deteriorates.
Furthermore, the electric capacitance detectable by the electric capacitance detection circuit 14 varies depending on environmental conditions, such as presence of other metallic substances, temperature, humidity, and environment surrounding the switch. Therefore, the electric capacitance is dependent on environmental factors and cannot be used to accurately detect the approach or contact of a user's finger.
Meanwhile, when it is possible to accurately detect a finger approaching irrespective of the magnitude of electric capacitance change, it may be desirable to employ a special control circuit and control software, as shown in Japanese Patent Application Laid-open Publication No. Hei 11-136116. However, a touch switch incorporating such a specialized control circuit and software is structurally complicated and more expensive.
On the other hand, according to the touch switch 200 shown in FIG. 2, the metallic layer 16 is exposed on the switch surface and a user's finger can directly contact the metallic layer 16. Compared to the touch switch 100 shown in FIG. 1, the touch switch 200 can produce a sufficiently large electric capacitance change. However, the touch switch 200 is disadvantageous in that the visual design of switch is restricted by the requirement that the metallic layer 16 be directly exposed on the surface of the switch.